In certain applications of unducted and ducted fans and propellers, it is desirable to be able to adjust the pitch of the blades either between runs or during runs. This can be done by manual adjustment, or by allowing the blades to self-adjust during operation.
Adjustment of fan or propeller blade pitch is known in the case of ordinary unducted fans and especially propellers. The mechanisms used to adjust the pitch are adequate as the propellers or fans are simply air movers and do not produce any significant pressure on the discharge side.
Many ducted fans are required to produce some form of head pressure, even in an axial situation where air travels between the blades in a path which is substantially parallel to the axis of rotation of the fan. An example of this is an axial flow compressor section of a gas turbine engine. These types of axial fans produce only small head pressures and to increase the pressure, need to be multi-staged. Radial or centrifugal fans produce a greater degree of head pressure than axial flow fans.
In my earlier impeller which is described in International patent application PCT/AU93/00581, I provided a pressure-boost impeller which had overlapping blades attached to a hub which could be of a frusto-conical shape. The blades were inclined relative to the rotational axis which produced a large throat area to reduce stall during rotation. The impeller could be used as an axial flow impeller while still producing appreciable head pressure and this was achieved by convergence between adjacent blades.
In use, my earlier impeller sat within a housing with the tips of the blades sweeping closely along the inner wall of the housing. The roots of the blades could be pivotally mounted to the hub to allow the pitch of the blade to be adjusted. Adjustability of the blades was desirable to maintain high efficiency. Because the hub could be curved in only one direction, rotating the blade to adjust its pitch, created a small but unwanted gap between the blade root and the surface of the hub and between the blade tip and the inner wall of the shroud. This small gap allowed fluid to pass back through the impeller, which reduced its efficiency.
As my impeller can be rotated at high speed, it is desirable to be able to mount the blades to the hub in an adjustable manner but in such a fashion that the blades do not break or separate from the hub due to inertial forces.
Australian patent 210289 discloses a radial flow impeller which can pressurise a gas by the standard technique of increasing the speed of the gas followed by a sudden change in the speed of the gas. The impeller includes a number of non-overlapping blades which are attached to a hub via a journalled disc. The hub has an annular portion which is curved in two directions and can be seen as being a portion of a sphere. The stated advantage is that this allows the blades to be twisted without creating a gap between the blade tip and housing or the blade root and hub. The top face of the disc is flat which does not present a problem with the fan of patent 210289; and indeed the patent does not offer any further teaching on this point.
Axial flow and mixed flow fans have a central hub portion containing the axis of rotation, and a number of blades attached to the central hub portion. In order to improve the efficiency of the fan, or vary its operating parameters, the pitch of the blades can be varied. This is typically achieved by having the blades mounted to the hub in such a manner to allow the blades to rotate or twist relative to the hub. Various complicated internal mechanisms are provided to allow the pitch of the blade to be varied.
A disadvantage with known arrangements is that for fans or impellers having a large number of blades, the internal mechanism is extremely complicated, while for fans and impellers having a relatively smaller diameter, and therefore a small hub portion, it is generally not possible to provide a robust and reliable mechanism to vary the pitch of the blades.
In international patent application PCT/AU93/00581, there is disclosed a pressure boost impeller having blades which are pivotally mounted to the hub. These blades can pivot freely and at high speed rotation of the impeller, blade flutter or other undesirable vibrations can occur. The international patent application does not describe any mechanism by which the pitch of the blades can be varied and held in position.
The present invention, in one form, has been developed to provide an impeller where the blades can be pivoted on the hub without resulting in undue gaps appearing between the blade and hub. The present invention can optionally include a simple and reliable system whereby the pitch of a plurality of blades mounted to a central hub portion can be varied and held in position.